This invention relates to processes in which nitrogen oxides are reacted or in which these oxides are undesirably emitted to the atmosphere. This invention has particular relationship to chemical reactions involving the absorption of NO.sub.x in arriving at the final product of a process and/or in the removal of NO.sub.x from process emissions.
An important adaptation of this invention is in the manufacture of nitric acid. This adaptation will be emphasized in this application in the interest of relating this invention to a specific industrial process so as to facilitate the understanding of this invention and its ramifications. It is to be understood that any adaptation of the principles of this invention to any other process is within the scope of equivalents thereof under the Doctrine of Equivalents as the Doctrine is interpreted in Uniroyal, Inc. v. Rudkin-Wiley Corp. 5USPQ 2d 1434 (C A FC1988) (at 1441).
In the conventional manufacture of nitric acid, nitrogen oxides derived from the oxidation of ammonia are absorbed in water to form the nitric acid. The principal nitrogen oxides that are involved in the absorption step are nitric oxide, NO, nitrogen dioxide, NO.sub.2, and the dimer of NO.sub.2, N.sub.2 O.sub.4. These oxides are herein referred to as NO.sub.x and are the oxides which are of significance to this invention.
The absorption of nitrogen oxides into water or aqueous nitric acid solutions is the process stage that requires the largest share of the total equipment volume and capital costs. NO.sub.2 is itself relatively water-insoluble, but gas-phase NO.sub.2 is in equilibrium with the dimer form, N.sub.2 O.sub.4, which is soluble. On the other hand, NO is relatively insoluble. To dissolve the NO, the practice in accordance with the teachings of the prior art is to oxidize the NO to NO.sub.2 in the gas phase by means of the residual oxygen in the process gas. Because the oxidation of NO in the gas phase is a kinetically slow reaction, and also because there is only 1 to 5% residual oxygen in the process gas, large amounts of absorber gas volume are required in prior art practice to provide the necessary gas-phase residence time. A nitric acid absorption train typically contains two or more absorption towers. As the NO.sub.x concentrations decrease through an absorber train, the decrease in gas-phase concentrations of NO and NO.sub.2 slows the reactions down to the point where it becomes more economical to discharge the residual NO.sub.x gases than to provide the extra absorber volume for conversion of the residual NO. The discharge of NO.sub.x creates an environmental problem.
The chemistry of NO.sub.x absorption in aqueous solutions in the manufacture of nitric acid is believed to take place by means of the following overall reactions: EQU 2NO.sub.2 (g) N.sub.2 O.sub.4 (g) (1) EQU N.sub.2 O.sub.4 (g)+H.sub.2 O (1) HNO.sub.3 (1)+HNO.sub.2 (1) (2) EQU 3HNO.sub.2 (1) HNO.sub.3 (1)+H.sub.2 O (1)+2NO (g) (3) EQU 2NO (g)+O.sub.2 (g)=2NO.sub.2 (g) (4)
Reaction (1) referred to above represents the gas phase dimerization of NO.sub.2 to form equilibrium amounts of the water-soluble tetroxide compound. As the N.sub.2 O.sub.4 dissolves and reacts with the water, Reaction (1) proceeds to the right. The absorption reaction, Reaction (2), results in the equimolar formation of nitric acid and nitrous acid in the liquid phase. Reaction (3) is the equilibrium disproportionation reaction of HNO.sub.2 in the liquid phase which generates NO. The so-generated NO rapidly desorbs to the gas phase because of its very limited liquid solubility. Reaction (3) represents both the disproportionation reaction of HNO.sub.2 and the desorption of NO to the gas phase.
Increases in HNO.sub.2 concentration in the liquid phase, which is a concommitant of progression of Reaction (2) to the right, will displace Reaction (3) to the right, generating additional NO. Reaction (4) represents the gas-phase oxidation of NO, which is believed to be the slow and controlling step in the overall set of the simplified absorption reactions, (1) through (4). Reactions (3) and (4) are not only the limiting reactions with respect to absorption rates and equipment size, they also are the stoichoimetric cause of incomplete NO.sub.2 absorption and the persistence of NO.sub.x in nitric acid plant tail gases.
In other processes involving uses and reactions of nitric acid, typical examples of which are organic nitrations, hydrometallurgical processes, precious metal refining and recovery and the like, the same reactions prevent the complete removal of NO.sub.x from the off-gases by absorption processes. Emission of nitrogen oxides from such processes, as Well as that of the NO.sub.x in nitric acid plant tail gases, is a serious atmospheric pollution problem, particularly because the nitrogen oxides contribute materially to smog and acid rain.
Catalytic reduction processes for NO.sub.x emission control have been used in accordance with the teachings of the prior art in the majority of nitric acid plants seeking to comply with NO.sub.x emission standards. Such processes typically use ammonia as the reducing gas and precious metal or other catalysts, as described in Welty, U.S. Pat. No. 4,164,546, and others. The wasteful consumption of ammonia and the elevated temperature requirements make catalytic reduction economically unattractive, and it is employed only in the absence of feasible alternatives.
Reaction (3) indicates that the formation and desorption of NO may be suppressed by the removal of HNO.sub.2 from its liquid phase solution in HNO.sub.3. Nitrous acid, HNO.sub.2, can be oxidized in accordance with the liquid-phase reaction: EQU HNO.sub.2 (1)+1/2 O.sub.2 (1)=HNO.sub.3 (1) (5)
Botton and Cosserat, in U.S. Pat. No. 4,372,935, attribute the problem of slow rates of NO.sub.x absorption to the inadequacy of Reaction (5) in the dilute gas concentration range of a nitric acid absorption train, and teach the use of extended gas-liquid contact time in the third absorber column of the plant train. Their data show that by increasing the overflow weir height on the last 20 trays of the third NO.sub.x absorber, the tail-gas NO.sub.x emission can be decreased. Doubling the overflow weir height reduces the tail-gas NO.sub.x concentration from 968 to 635 ppmv (parts per million by volume) and increasing the weir height 4.4 times, decreases the NO.sub.x to 376 ppmv. Essentially, the Botton teaching is to achieve increased mass transfer and liquid dwell time by increasing the overflow weir height. But, notwithstanding the substantial increased cost and complexity which is involved, the method is ineffective.
Herbrechtsmeier, Steiner and Vilczek, U.S. Pat. No. 4,309,396, teach the partial reduction of NO.sub.x emissions, for example, from 3,300 ppmv to 2,000 ppmv, by large increases in total gas pressure and liquid/gas ratio in an NO.sub.x absorber of a nitric acid plant. The reduction is not sufficient to meet the standards for NO.sub.x emission reduction imposed by most environmental laws, which require an exhaust concentration of approximately 200 ppmv.
Janiczek, in Polish No. 61061, teaches the treatment of acid taken from a nitric acid plant absorber train with oxygen in a countercurrent flow contactor. Reus, Wewer and Jungen, U.S. Pat. No. 4,419,333, teach the use of air to oxidize NO absorbed in 15-50 weight % HNO.sub.3. It is conjectured that to obtain dissolved oxygen for Reaction (5), it is necessary to transfer oxygen to the liquid phase: EQU 1/2 O.sub.2 (g)=1/2 O.sub.2 (1) (6)
Both Janiczek and Reus employ "open-ended" systems for liquid-gas contacting, in which the gas flows continuously through the contactor and therefore serves as an acceptor and infinite sink for gases stripped from the liquid. This apparatus has not been effective.
Fauser, in Chemical and Metallurgical Engineering, Vol. 39, pp. 430-432, August, 1932, describes a method for directly manufacturing 98% HNO.sub.3 in which a mixture of dilute HNO.sub.3 and N.sub.2 O.sub.4 is fed to an autoclave, the temperature is raised to 70.degree. C., and then oxygen at a pressure of 50 atmospheres is introduced. Even under these extreme conditions, four hours were required to complete the reaction. Because of the batch nature of the operation, and the long time for reaction completion, this process is not suitable for continuous NO.sub.x production, i.e., continuous oxidation and absorption and removal of product.
It is an object of this invention to overcome the disadvantages and drawbacks of the prior art, and to provide a method for continuous production of nitric acid which shall be more efficient than prior art methods. It is another object of this invention to provide a method for the enhanced absorption of NO.sub.x from nitric acid plant tail gases and other industrial NO.sub.x emission sources. Another object of this invention is to provide apparatus for practicing this method. More generally stated, it is an object of this invention to provide a new method and apparatus for the enhanced absorption of nitrogen oxides and particularly, nitric oxide, in industrial processes in a practical manner at comparatively low cost, and to control emissions of nitrogen oxides from such processes.